Electro-pneumatic positioners are the most commonly used type of positioners. Their pneumatics and mechanics are similar to those of pneumatic positioners. However, electro-pneumatic positioners feature a directly attached upstream converter module that converts the reference input of an electrical signal (4 to 20 mA) into a pneumatic signal between 0.2 and 1 bar.
Digital positioners link the control system to the valve. A microprocessor helps to control the position until the target value is reached by comparing the signal of the valve position with the input variable and calculating the switch response of the outlet valves. Available communication profiles include HART protocol, Foundation Fieldbus and Profibus PA.
The advantages of digital positioners:
- reduced air consumption
- automatic commissioning (adjustment of zero and span)
- continuous control loop monitoring
- adjustable control speed
- digital communication via HART protocol, Foundation Fieldbus, Profibus
In pneumatic positioners, the reference variable is a pressure signal between 0.2 and 1 bar. The positioner receives a pneumatic input signal and modulates the supply pressure of the control valve actuator to keep the position proportional to the input signal. The mode of operation follows the mechanical force balance principle.
Analogue I/P valve positioners convert electrical signals into pneumatic signals. They are mostly used for the continuous control of control valves with a direct current signal between 4 and 20 mA. The integrated electronics compare the target value of the current with the electric variable of the stroke position, thus regulating the E/P converter.
Positioners are essential for achieving precise control results. They are used to correctly position and retain the valve stem despite external influences, for instance during ventilation of the actuator chamber or in case of perturbations such as frictional forces (e.g. at the packing gland).
The input signal that comes from a universal controller or from control systems as an electrical or pneumatic signal is converted to an output signal by the positioner. This signal determines the required position. Position repeaters or resistance transmitters allow for continuous signalling. In addition, they indicate the current stroke position of the control valve.
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